By using OFDM (orthogonal frequency division multiplexing) as a multiplexing method, a frequency-selective channel is converted into parallel frequency flat sub-channels. Sub-carriers have a minimum frequency separation necessary to maintain orthogonality of the corresponding time domain waveforms.
OFDM is a block modulation scheme where a block of N information symbols is transmitted in parallel on N sub-carries. The duration of an OFDM symbol is N times larger than that of a single-carrier system.
An OFDM modulator can be implemented as an inverse discrete Fourier transform (IDFT) on a block of N information symbols followed by an analog-to-digital converter (ADC). To decrease the effects of intersymbol interference (ISI) caused by channel time spread, each block of N IDFT coefficients may be preceded by a cyclic extension (prefix or postfix) or a guard interval. In a receiver, fast Fourier transform (FFT) may be used in signal processing.
In the receiver of a communication system using OFDM, one of the most challenging tasks is the synchronization of an OFDM signal. The synchronization requires finding symbol timing (a time offset) and carrier frequency offset. In order to find symbol timing, the beginning of a symbol has to be estimated. A carrier frequency has to be synchronized very accurately; otherwise there will be loss of orthogonality between sub-symbols. OFDM systems are very sensitive to carrier frequency offsets since they can only tolerate frequency offsets which are a fraction of the frequency spacing between sub-carriers without degradation in system performance.